"Additional examination of the fragments of the Tu-134 aircraft which crashed on Tuesday ... has revealed traces of hexogen," an FSB spokesman said by telephone on Saturday.
The FSB said on Friday that hexogen, more widely known as RDX, had been found in the wreckage of the first plane which crashed on Tuesday in southern Russia.
Investigators had been pursuing leads linked to terrorism in the crashes before Sunday's election, certain to return a pro-Moscow president in Chechnya.
Interfax news agency quoted FSB spokesman Sergei Ignatchenko as saying Russia was "studying international experience in fighting terrorism in air transport", particularly the system used in Israel.
As the investigation proceeded and fragments of wreckage were removed from crash sites, Russia's transport minister
toughened security measures and vowed to prevent any recurrence.
Igor Levitin said his concern was to ensure safe air travel. Safety measures, previously undertaken solely by airports, would now be shared with the Interior Ministry.
"We want to toughen all requirements in terms of cargo and baggage ... Passengers must be made to feel that everything is in order once they are seated in an aircraft"
Russia's transport minister
"From today, they [Interior Ministry officials] are being included in teams conducting searches," said Levitin who was ordered by President Vladimir Putin to head a commission to investigate the crashes.
"We want to toughen all requirements in terms of cargo and baggage ... Passengers must be made to feel that everything is in order once they are seated in an aircraft."
Authorities understood the crashes were "an extraordinary event ... We must look thoroughly into this to understand what happened and take measures to ensure it does not happen again."
Investigators have carefully avoided any suggestion that
Chechen fighters were behind the crashes.
But Russian media have speculated that two passengers, believed to be Chechen women, blew up the planes in the run-up to Sunday's election.
A Tu-134 aircraft crashed on a flight from Moscow to Volgograd, while a few minutes later a Tu-154 came down en route from the capital to the Black Sea resort of Sochi.
NTV television showed investigators wrapping up search operations on the Tu-134 crash site. Television also showed pallbearers in black passing through Volgograd streets with coffins draped in wreaths. Mourners were shown weeping at a funeral in Sochi.
Alu Alkhanov (L) is almost sure
to win Sunday's poll
Russian media said investigators were trying to determine whether two women with Chechen names were linked to the crashes. The daily Izvestia reported that the brother of one woman had been seized by Russian forces in Chechnya three years ago.
Chechnya's Muslim fighters, who denounce the presidential election as a farce, have staged spectacular attacks to press their independence drive.
Moderate separatists accuse Russia's special forces of spreading misinformation and deny any connection with a group which claimed responsibility for the crashes on Friday.
Chechen Interior Minister Alu Alkhanov, backed by the Kremlin, is almost sure to win Sunday's poll, called to replace a president assassinated in May. He faces six obscure rivals.